As long as you have a Blackberry Services account set up with your curve (you can already use BBM and receive emails etc) you can use it through your Playbook too. You will first need to install the Blackberry Bridge App on your phone (it's free from Appworld).
The Playbook connects to the curve with Bluetooth and mirrors your BBM, Messages, Calender, Memos etc. When not to the phone, none of this will be available or stored on the Playbook.
The Playbook doesn't have it's own PIN, you basically use it as an extension of your Blackberry Handset. It has been criticised for this but I think it's a good feature.
You can also use the Playbook to browse and go online through the 3G connection on your phone when there is no WiFi available (check you won't get charged for data usage from your Mobile Contract, as long as you have unlimited Blackberry Services you should be OK).
There is a Major software update coming to the Playbook in Febuary which is promising a stand alone Email App amongst other things so you won't need to use Bridge to access your messages. I believe a separate PIN for BBM is in the pipeline but until then you'll need to have your Curve handy to use it.
I've never heard of anyone with a Blackberry Handset and a Playbook complaining about this, it's only a problem if you have the tablet but not the phone. (although you can access Email though the web browser easy enough).
The Playbook has received loads of criticism, some of it deserved, but after recent software updates and the availability of more and more apps, most of the people that own them really love them. They're available at a great price right now. I bought mine when they were only a little cheaper than the iPad 2 and I really don't regret it one bit. I much prefer using it to my friends Apple products.
ENJOY!